AUG 2014

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

Issue link: https://digital.eyeworld.org/i/357599

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Page 67 of 82

W hen differentiating between peristaltic and venturi pumps, it is important to review the goals of proper fluidic relationships. The objective is to have sufficient power to remove the cataract while maintaining a stable anterior chamber and min- imizing corneal damage and heat production. Another crucial concept when discussing phacoemulsification pumps is inflow and outflow. Usu- ally inflow is defined by gravity and hence, bottle height. The higher the bottle, the greater the difference between the height of the bottle and the eye, leading to a higher inflow. Inflow is accompanied by a programmable pump. Some newer phacoemulsification machines have substituted an active pump system rather than relying on gravity. Outflow is defined by vacuum, aspiration, tubing size, and incision architecture. Aspiration flow rate may seem like an amorphous factor, but it actually refers to how much and how fast fluid is removed. It is what causes nuclear pieces to be attracted to the phaco tip. In turn, vacuum is Preventing surge Surge occurs when a fragment that is occluding the port is suddenly aspirated. When the fragment is occluding the phaco tip, the tubing collapses due to negative pressure. When the fragment clears, there is a sudden expansion of the tubing causing a rapid rush of fluid into the tubing and subsequent flattening of the anterior chamber. Surge is often associated with the rigidity of the tubing. The more pliant the tubing, the higher the likelihood of creating surge. To help control surge, the surgeon can increase the inflow into the eye with a higher bottle height, lower aspiration flow rate, and lower vacuum preset. Today's phaco tech- nology often will have built-in aids to help avoid surge. These include digitally controlled and automated infusion systems, rigid tubing, and a bypass port to allow some flow even when the tip is occluded. Phaco device settings; avoiding wound burn Altering the fluidics for the density of the cataract can be advantageous. For a venturi pump system, a typical setting for a moderate cataract is a bottle height of 135 cm, power of 40, vacuum of 275, duty cycle of 40%, and 30 pulses per second (pps). In contrast, with a dense lens, the bottle height remains at 135 cm, but the power can be increased to 60, vacuum increased to 290, duty cycle increased to 70%, with 70 pps. If the patient has weak zonules, I adjust my settings to a bottle height of 50 to 75 cm, a power of 40, a vacuum of 35, 50 to 70 pps depending on density, and a duty cycle of 50%. With weak zonules, the entire diaphragm of the capsular bag tends to trampoline up and down, so the goal is to stabilize the chamber. By doing so, this can prevent the vitreous from prolapsing anteriorly and hopefully prevent the need for a vitrectomy. As inflow decreases with a lower bottle height, I decrease the vacuum and decrease power to about 40. Although wound burn is less common with today's advanced phaco technology, it still is some- thing every surgeon tries to avoid. A few surgical pearls to prevent a wound burn are to aspirate some of the viscoelastic material before beginning to sculpt or vacuum the lens and to ensure proper incision architecture. Make sure the incision size is appropriate, the entry into the anterior chamber is square, and avoid torquing the wound when using the phaco handpiece. Paying close attention to these types of small details are the basics of phaco fundamentals. Dr. Henderson is in private practice at Ophthalmic Consultants of Boston, and clinical professor of ophthalmology at Tufts University School of Medicine. She can be contacted at bahenderson@eyeboston.com. Bonnie An Henderson, MD by Bonnie An Henderson, MD Understanding peristaltic versus venturi pumps Peristaltic pump Source: Bausch + Lomb the negative pressure that keeps the nuclear fragments on the phaco tip; the higher the vacuum, the greater the holding force. A peristaltic pump uses flexible tubing compressed against a rigid rotor by a series of rollers on the pump head. This creates a rela- tive vacuum when the aspiration port is occluded. There are several advantages to peristaltic pumps. The surgeon can set vacuum limits independent of flow and can allow a different amount of flow but a great- er amount of vacuum. Peristaltic pumps allow for moderate flow with low vacuum to increase thermal safety during sculpting. However, with peristaltic pumps, you have to have a clear understanding of how flow and vacuum work, as they can be changed inappropriately in novice hands. With the venturi pump, the flow and vacuum must work together so there is only one variable to change. In a venturi system, the tip does not have to be occluded to create vacuum. The movement of the phaco port can be minimized and without needing to chase pieces. The disadvantage is that you have to be more cautious of what else is around your tip at the time. When the vacuum is engaged, not only will the nuclear fragment be attract- ed to the tip, the capsule will also be attracted. With today's efficient, advanced systems, you will notice the rapid movement of the pieces toward the tip. This CME supplement is supported by unrestricted educational grants from Alcon and Bausch + Lomb. Venturi pump

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